In the continuous drying of resinous materials, it is important to obtain a high efficiency of the process while at the same time maintaining certain parameters of the drying process at a level which would not adversely affect the quality of the substance being dried. If is especially important to carry out the drying process at temperatures which would not result in degradation of the polymeric materials.
Driers with dual counter-rotating shafts are suggested in the art. For example, U.S. Pat. No. 4,226,973 to Malo, et al. suggests a horizontal, agitated, twin-shaft, jacketed vessel with adjustable paddles mounted on the shafts. Hollow shafts may contain heating elements to provide temperature control. This apparatus can perform drying, crystallization, and solid phase polymerization for upgrading prepolymer particles.
Another example of a drying apparatus with hollow shafts is shown in U.S. Pat. No. 3,678,596 to Kubo for continuous drying of slurry materials. This dryer includes a pair of hollow shafts rotating in predetermined opposite directions and a large number of paddles radially disposed on each shaft and engaging with each other as the two shafts rotate. A horizontal cylindrical vessel surrounds the shaft and jacket. Means are provided for blowing hot gas into the jacket and keeping the material in the co-current or parallel flow through the bulkhead. Also the dryer includes means for combining the co-current of the hot gas exhausted from the holes of the shafts end and openings of the jacket end and means for heating by direct contact with the material in the countercurrent. However, the drying process aimed for in the present invention differs significantly therefrom. For instance, the use of relatively low temperatures is important to the success of the present invention. On the other hand, see the example of Kubo at column 3, line 51, through column 4, line 14, wherein the high temperatures of the inlet gas of 235.degree. C. at the inlet 133.degree. C. at the shaft outlet and 94.degree. C. at the vessel outlet are employed.